The proposed research program is organized to examine, primarily by advanced electron microscope techniques, the cell biology of the alveolar-capillary unit of the lungs. Two of the component cells, namely the capillary endothelial cells and the type II alveolar epithelial cells appear to carry out specific metabolic activities that are independent of the physical exchange of gases between blood and air. The endothelial cells contain hydrolase enzymes capable tt metabolizing circulating hormonal substances, inactivating some (e.g. bradykinin) while activating others0(e.g. angiotensin I). Our previous studies indicate that the enzymes responsible for these metabolic events are situated on or very near the luminal surface of the endothelial cells. Using endothelial cells in situ and isolated in culture, we plan to coordinate ciochemical, fine structural and cytochemical studies. The overall objective of our research is to develop an improved understanding of the alveolar-capillary unit in its role in the "endocrine" and non-ventilatory functions of the lungs.